The present invention relates generally to afterburners for aircraft gas turbine engines, and in particular to a heat shield for protecting an afterburner fuel injection tube that is readily replaceable from inside an engine duct.
Afterburning or reheating is one method of augmenting the basic thrust of a turbine engine. An afterburner increases thrust by adding thermal energy to a stream of turbine exhaust gas and engine bypass air located aft of the core engine. The afterburner includes several fuel injection tubes, or spray bars, having multiple spray orifices for dispensing fuel into the stream of gas. The tubes frequently extend radially inwardly from an outer wall of the engine duct. The afterburner also includes flame stabilization devices, known as flameholders, for creating regions of reduced gas velocity to facilitate effective combustion.
Components of the engine duct and afterburner are frequently cooled for protection from exposure to high temperatures and to extend useful lifetime. Cooling is provided by air that bypasses the core engine, known as fan air, which flows through one or more stages of the engine""s fan section, or alternatively by compressor bleed air. Cooling air is typically conveyed in an annular bypass duct formed between the cylindrical outer wall and a liner spaced radially inwardly from the outer wall. The liner is a cylindrical shell which functions as a barrier to isolate hot exhaust gas from the cooling air. The cooling air is distributed from the bypass duct to the afterburner, surface of the liner, and exhaust nozzle.
Each of the fuel injection tubes is typically protected by a heat shield. The heat shield is a housing for enclosing the tube and receiving cooling air from the bypass duct for delivery along the tube. A plurality of window openings are spaced along the heat shield for alignment with orifices of the tube to permit fuel to be sprayed from the tube into the gas stream. Flameholders may be mounted on the heat shield or, alternatively, mounted in the duct separate from the fuel injection tubes.
Heat shields are periodically replaced due to damage, thermal cyclic fatigue, or accumulation of coking residue. Ideally a heat shield should be replaceable in a short time so that aircraft down-time is minimized. Unfortunately, it requires many hours to replace some types of heat shields because they are not removable from a position within the engine duct. Either the heat shield is not installable over the fuel tube from inside the duct, or an attachment between the heat shield and its supporting structure is not accessible from inside the duct. Accordingly, in order to replace these heat shields, maintenance personnel must first remove the engine from the aircraft. That necessitates disconnecting all electrical cables, fuel lines, and hydraulic lines between the engine and airframe, and re-connecting them upon completion of the maintenance action. These steps are time consuming and expensive.
Another type of heat shield, shown in U.S. Pat. No. 5,335,490, is installable and removable from inside the engine duct but still requires disconnecting the fuel injection tubes. That type of heat shield has a support attachment with an axially extending nose connector. During installation or removal, the heat shield must be moved transverse to the axis of the fuel tube in order to engage/disengage the connector. Such transverse motion is impossible while the fuel tube is installed due to interference with internal structure that is part of every heat shield. Accordingly, the fuel tube must be removed and subsequently reinstalled, requiring a time-consuming fuel system leak test.
When a heat shield is mounted in position in the engine duct, it must be firmly attached with a sturdy, reliable connection between the heat shield and its support. Unfortunately, some types of heat shields have a drawback in that the connection is at a single point, with only one fastener. As a result, the heat shield is subject to slippage when exposed to acoustical vibrations in the engine or to high loads, particularly due to forces transverse to the shield when the heat shield is at a 3 or 9 o""clock position in the engine duct and the aircraft experiences a hard landing.
In general, a heat shield of the present invention is for protecting a fuel injection tube of a turbine engine afterburner section. The afterburner section is of the type having a cylindrical outer wall and a liner positioned in spaced relation from the outer wall and generally defining a boundary between a core duct for flow of turbine exhaust gas and an annular bypass duct for flow of cooling air. At least one fuel injection tube extends from the outer wall inwardly to the core duct. The tube has a longitudinal axis and is mounted so that a first portion of the tube is located in the bypass duct and a second portion of the tube is located in the core duct. A strut is positioned generally between the outer wall and the liner and enclosing the first portion of the fuel injection tube. The heat shield comprises a housing having an elongate internal channel therein open at one end for allowing the housing to be telescoped over the tube to a mounting position in which the second portion of the tube extends longitudinally in the channel. A mounting system on the housing adjacent the open end of the channel attaches the housing to the strut when the housing is in the mounting position without moving the housing transverse to the longitudinal axis of the fuel tube so that the housing can be installed in the afterburner section without moving or disconnecting the fuel tube.
In another aspect, a support strut of the present invention supports a heat shield in a turbine engine afterburner section. The afterburner section is of the type having a cylindrical outer wall, a liner positioned in spaced relation from the outer wall and generally defining a boundary between a core duct for flow of turbine exhaust gas and an annular bypass duct for flow of cooling air. At least one fuel injection tube extends from the outer wall inwardly to the core duct. The tube is mounted so that a first portion is located in the bypass duct and a second portion is located in the core duct. The heat shield has a housing for enclosing the second portion of the tube and has a head with a generally flat mounting surface. The support strut comprises a body mountable in the bypass duct of the afterburner section. The body has an outer end for attachment to the outer wall, an inner end, and a passage extending through the body from its outer end to its inner end for receiving the fuel tube. The inner end has a cavity therein for receiving the head of the heat shield, the cavity being sized and shaped for a close fit of the head in the cavity. The body has a generally flat mounting surface in the cavity for engagement by the flat mounting surface of the head whereby the head may be placed in the cavity to mount the heat shield on the strut with the mounting surfaces in face to face engagement to comprise a solid connection between the head and the body.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.